In conventional vertical multi-articulated robots having a turning body rotatable about a first rotational axis, an upper arm is arranged to rotate about a second rotational axis intersecting the first axis at a right angle, an upper arm drive motor is attached to the turning body, and a forearm drive motor is attached to the upper arm. These motors are arranged coaxially with the rotational axis of the upper arm and project laterally in opposite directions from an interconnecting portion between the upper arm and the turning body. Output shafts of the upper arm drive motor and the forearm drive motor are connected through reduction gears and power transmitting mechanisms to rotatable shafts of the upper arm and forearm, respectively.
In the above-mentioned robot, generally, a moment of force about the second axis due to gravity is increased, and accordingly, the load of the upper arm drive motor is increased, as the upper arm moves from a vertical attitude where the tip thereof is orientated immediately above to a horizontal attitude. Accordingly, it is necessary to provide a high power motor as an upper arm drive motor. In order to decrease the load of the upper arm drive motor due to a moment of force, conventional vertical multi-articulated robots are provided with a counter-balancing mechanism using a force of a spring provided between the turning body and the upper arm. However, the use of the counter-balancing mechanism complicates the structure of the robots and increases manufacturing costs thereof.
In turn, the conventional vertical multi-articulated robot has a workable region which is defined, generally, within an angular range of 90 degrees to the right and to the left, respectively, relative to a reference position of the base about the first rotational axis of the turning body. However, in the conventional vertical multi-articulated robot, the upper arm drive motor and the forearm drive motor projecting laterally in opposite directions from the interconnecting portion between the upper arm and the turning body move into the workable region of the robot, as the turning body rotates. Accordingly, the workable region of the robot is considerably reduced due to the necessity of ensuring therein a space permitting these movements of the upper arm drive motor and the forearm drive motor.